Composite feedstock strips for additive manufacturing and methods of forming thereof

ABSTRACT

Provided are composite feedstock strips for additive manufacturing and methods of forming such strips. A composite feedstock strip may include continuous unidirectional fibers extending parallel to each other and to the principal axis of the strip. This fiber continuity yields superior mechanical properties, such as the tensile strength along strip&#39;s principal axis. Composite feedstock strips may be fabricated by slitting a composite laminate in a direction parallel to the fibers. In some embodiments, the cross-sectional shape of the slit strips may be changed by reattributing material at least on the surface of the strips and/or by coating the slit strips with another material. This cross-sectional shape change may be performed without disturbing the continuous fibers within the strips. The cross-sectional distribution of fibers within the strips may be uneven with higher concentration of fibers near the principal axis of the strips, for example, to assist with additive manufacturing.

CROSS-REFERENCE TO RELATED APPLICATION

This application is also a continuation-in-part of U.S. application Ser.No. 14/835,323, entitled “COMPOSITE FEEDSTOCK STRIPS FOR ADDITIVEMANUFACTURING AND METHODS OF FORMING THEREOF,” filed on 25 Aug. 2015,(Attorney Docket No. 15-0809_BNGCP064US), which is incorporated hereinby reference in its entirety for all purposes.

BACKGROUND

Additive manufacturing is a process of forming a three-dimensional (3D)object adding layers of material, such as plastic and metal. The processoften relies on computer systems and, more specifically, on computeraided design (CAD) to design each layer and the overall layup process.Additive manufacturing is particularly attractive for complex, lowvolume parts that are frequently used in, for example, aerospaceapplications. Stereo lithography (SLA), selective laser sintering (SLS)and fused deposition modeling (FDM) are currently three primary methodsused to make additively manufactured components. Typically, neat resins,which are materials without any structural supports (e.g., fibers), areused for this purpose. Incorporating structural supports into additivemanufacturing feedstock proved to be difficult and generally limited tosmall particles and short fibers. However, these types of structuralsupports do not yield mechanical properties associated with continuousfibers. Furthermore, current techniques used for fabricating compositefeedstock, such as extrusion, may cause voids and other defects in thefeed stock. Finally, extrusion and other like techniques of fabricatingcomposite feedstock are prone to clogging with structural supports.

SUMMARY

Provided are composite feedstock strips for additive manufacturing andmethods of forming such strips. A composite feedstock strip may includecontinuous unidirectional fibers extending parallel to each other and tothe principal axis of the strip. This fiber continuity yields superiormechanical properties, such as the tensile strength along strip'sprincipal axis. Composite feedstock strips may be fabricated by slittinga composite laminate in a direction parallel to the fibers. In someembodiments, the cross-sectional shape of the slit strips may be changedby reattributing material at least on the surface of the strips and/orby coating the slit strips with another material. This cross-sectionalshape change may be performed without disturbing the continuous fiberswithin the strips. The cross-sectional distribution of fibers within thestrips may be uneven with higher concentration of fibers near theprincipal axis of the strips, for example, to assist with additivemanufacturing.

Provided is a method of forming coated composite feedstock strips foradditive manufacturing. In some embodiments, the method comprisesslitting a sheet into composite feedstock strips and coating an outersurface of the composite feedstock strips. For example, the outersurface may be coated with a material comprising a second resin. Thiscoating process forms a coating layer over the surface the compositefeedstock strip. This combination of the coating layer and the compositefeedstock strip is referred to as a coated composite feedstock strip.

The sheet used for slitting may comprise a first resin and continuousfibers extending parallel to each other within that sheet. The slittingmay be performed along the direction parallel to the continuous fiberswithin the sheet thereby preserving continuity of the fibers. Thecoating may be performed using a cross-head extrusion coating techniqueor any other suitable technique, such as powder coating andsolution-based coating technique

In some embodiments, the distribution of the continuous fibersthroughout the cross section of the composite feedstock strips isuniform prior to coating these strips. This fiber distribution ispreserved during slitting. As such, the distribution of the continuousfibers throughout the cross section of the sheet used for slitting maybe also uniform. However, once the slit strips are coated, thiscross-sectional distribution changes since no continuous fibers may beused in the coating materials, e.g., the second resin. In someembodiments, the second resin may include different type of fibers orother types of fillers or may be substantially free from any fibers orfillers. For example, the concentration of non-resin components in thecoating material may be less than 5% by volume or even less than 1% byvolume.

Alternatively, the coating material may include a filler selected fromthe group consisting of fibers, particles, and flakes. For example, thefiller may comprise discontinuous fibers, which are different from thecontinuous fibers of the sheet and later of the composite feedstockstrips at least based on their aspect ratio. The filler may be selectedfrom the group consisting of a heat sensitive additive, a mineralreinforcement, a thermal stabilizer, an ultraviolet (UV) stabilizer, alubricant, a flame retardant, a conductive additive, and a pigment.

In some embodiments, one of the first resin and the second resincomprises one or more materials selected from the group consisting ofpolyethersulfone (PES), polyphenylenesulfide (PPS), polyetheretherketone(PEEK), poiyetherketoneketone (PEKK), polyetherimide (PEI), andthermoplastic polyimide (TPI). The first resin and the second resin aresame. For example, the first resin and the second resin may be bothpolyetherketoneketone (PEKK).

In some embodiments, the thickness of the coating layer formed on theouter surface of the composite feedstock strips is uniform. This type ofcoating may be also referred to as a conformal coating. The thicknessvariation may be less than 20% or even less than 10%. In theseembodiments, the cross section of the coated composite feedstock stripmay represent a scaled up variation of the cross section of thecomposite feedstock strips prior to coating.

In some embodiments, the concentration of the continuous fibersthroughout the cross section of the composite feedstock strips is atleast about 40% by volume prior to coating these strips. Since fibersare not added or removed during slitting, the fiber concentration of thesheet slit into the continuous strips may be the same. Thisconcentration may be controlled during fabrication of the sheet, forexample, through selection of plies for lamination.

In some embodiments, the cross section of the composite feedstock stripsor, more specifically, the cross-sectional profile remains same whilecoating the outer surface of the composite feedstock strips with thematerial. In other words, the coating process may not disturb thecomposite feedstock strips.

In some embodiments, the cross-sectional profile of the uncoatedcomposite feedstock strips is selected from the group consisting of arectangle, a square, and a trapezoid. The cross-sectional profile of thecoated composite feedstock strips is selected from the group consistingof an oval, a circle, a rectangle, a square, and a rounded cornerrectangle, and a rounded corner square.

In some embodiments, the method further comprises forming the sheet usedfor slitting. This operation is performed prior to slitting the sheetand may involve forming a layup comprising fiber containing pliesfollowed by laminating this layup. In some embodiments, all sheets ofthe layup are fiber containing plies. In these embodiments, thevolumetric fraction of the continuous fibers within the laminated sheetmay be constant. Alternatively, the layup may be formed from one or morefiber containing plies as well as one or more of resin plies. The resinplies may be free from fibers or any other fillers. In these alternativeembodiments, the volumetric fraction of the continuous fibers within thelaminated sheet varies throughout the thickness of the laminated sheet.For example, the volumetric fraction of the continuous fibers within thelaminated sheet is greater at a center of the laminated sheet along thethickness of the laminated sheet than at one of surfaces of thelaminated sheet.

In some embodiments, prior to coating the outer surface of the compositefeedstock strips, the method may involve changing a cross-sectionalprofile of each of the composite feedstock strips. For example, theuncoated composite feedstock strips may include surface portions freefrom continuous fibers and materials from these surface portions may beredistributed thereby forming a new cross-sectional profile.

Provided also is a coated composite feedstock strip for additivemanufacturing. In some embodiments, the coated composite feedstock stripcomprises a composite feedstock strip and a coating layer disposed onthe outer surface of the composite feedstock strip. The compositefeedstock strip comprises a first resin and continuous fibers extendingparallel to each other within the sheet. The coating layer may beforming a complete or a partial shell around the composite feedstockstrip.

These and other embodiments are described further below with referenceto the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a process flowchart corresponding to a method of formingcomposite feedstock strips for additive manufacturing, in accordancewith some embodiments.

FIG. 1B is a process flowchart corresponding to another method offorming composite feedstock strips including coating layers, inaccordance with some embodiments.

FIG. 2A is a schematic cross-sectional representation of a layupincluding multiple resin plies and fiber containing plies, in accordancewith some embodiments.

FIG. 2B is a schematic perspective representation of a portion of thelayup shown in FIG. 2A illustrating orientations of fibers in one of thefiber containing plies, in accordance with some embodiments.

FIG. 2C is a schematic cross-sectional representation of a layupincluding only fiber containing plies, in accordance with someembodiments.

FIG. 2D is a schematic cross-sectional representation of a layupincluding resin plies and fiber containing plies such that outer pliesof the layup are fiber containing layers, in accordance with someembodiments.

FIG. 3A is a schematic cross-sectional representation of a laminatedsheet formed from the layup shown in FIG. 2A, in accordance with someembodiments.

FIG. 3B is a schematic plot showing volumetric fraction of fibers as afunction of location along the thickness of a laminated sheet, inaccordance with some embodiments.

FIG. 3C is another plot showing a constant volumetric fraction of fiberswithin the laminated sheet, in accordance with some embodiments.

FIG. 3D is a schematic perspective representation of the laminated sheetshown in FIG. 3A illustrating slitting directions, in accordance withsome embodiments.

FIG. 4A is a schematic perspective representation of composite feedstockstrips formed from the laminated sheet shown in FIG. 3D, in accordancewith some embodiments.

FIG. 4B is a schematic cross-sectional representation of a compositefeedstock strip, in accordance with some embodiments.

FIG. 4C is a schematic perspective representation of the compositefeedstock strip shown in FIG. 4B, in accordance with some embodiments.

FIG. 5A is a schematic representation of a liquefier used for changingthe cross-sectional profile of composite feedstock strips, in accordancewith some embodiments.

FIG. 5B is a schematic cross-sectional representation of a compositefeedstock strip entering the liquefier shown in FIG. 5A, in accordancewith some embodiments.

FIG. 5C is a schematic cross-sectional representation of the compositefeedstock strip leaving the liquefier shown in FIG. 5A, in accordancewith some embodiments.

FIGS. 5D-5F are schematic cross-sectional representations of differentexamples of uncoated composite feedstock strips.

FIGS. 5G-5K are schematic cross-sectional representations of differentexamples of coated composite feedstock strips.

FIGS. 5L-5N are schematic cross-sectional representations of a compositefeedstock strip at different fabrication stages when the compositefeedstock strip changes its cross-sectional profile prior to coating.

FIGS. 5O-5Q are schematic cross-sectional representations of a compositefeedstock strip at different fabrication stages when the compositefeedstock strip changes its cross-sectional profile after coating.

FIGS. 6A and 6B are schematic cross-sectional representations of a bentcomposite feedstock strip, in accordance with some embodiments.

FIG. 7A is a schematic representation of an apparatus used for forming alaminated sheet, in accordance with some embodiments.

FIG. 7B is a schematic representation of an apparatus used for forming acoated composite feedstock strip, in accordance with some embodiments.

FIG. 8A is a photo of a cross-section of a composite feedstock stripused in a test.

FIG. 8B is a plot of a coating volume fraction in a coated feedstockwith a circular coating on a square laminate core as a function of thelaminate thickness and coating thickness.

FIG. 8C is a plot of the fiber content in a coated feedstock as afunction of the laminate thickness, coating thickness, and coating crosssectional shape.

FIG. 9 is a block diagram of aircraft production and service methodologythat may utilize end effectors described herein.

FIG. 10 is a schematic illustration of an aircraft that may includecomposite structures described herein.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toprovide a thorough understanding of the presented concepts. Thepresented concepts may be practiced without some or all of thesespecific details. In other instances, well known process operations havenot been described in detail so as to not unnecessarily obscure thedescribed concepts. While some concepts will be described in conjunctionwith the specific embodiments, it will be understood that theseembodiments are not intended to be limiting.

Introduction

Many applications, such as aerospace, require parts with complexgeometries yet low production volumes. While many techniques suitablefor high production volumes, such as molding, have been developedovertime, these techniques are cost prohibitive and often do not produceparts with needed characteristics. Additive manufacturing has recentlygained a lot of popularity in attempts to fill this void. However, manystructural requirements (e.g., strength of fabricated components) cannotbe easily achieved with current additive manufacturing techniques. Forexample, incorporating structural supports, such as fibers or particles,into additive manufacturing feedstock has been a major challenge. Evensmall fibers and particles tend to clog extruding nozzles whenattempting to directly form feedstock with small cross-sectionalprofiles. Yet, small profiles are essential for fabricating parts withcomplex geometries, tight dimensional tolerances, and/or smooth surfacefinish.

One area of particular interest for composite materials in general andfor composite parts formed using additive manufacturing in particular isusing continuous fibers. Continuous fibers provide high strengths levelsin the direction of the fiber. For example, a composite feedstock stripformed from a polyaryletherketone (PAEK) resin and filled with 30% byvolume of chopped carbon fibers may have a tensile modulus of about 3million pounds per square inch (MSI). At the same time, a compositefeedstock strip formed from the same resin and filled with 35% by volumeof continuous carbon fibers may have a tensile modulus of greater than10 MSI. Furthermore, composite parts produced using continuous fiberfeedstock are expected to have roughly six times the strength and tentimes the stiffness of comparable unreinforced parts currently produced.

However, incorporating continuous fibers into additive manufacturing iseven more challenging than incorporating short fibers and particles.Current additive manufacturing techniques are not simply capable ofproducing composite feedstock strip with continuous fibers at commercialscales. Handling of continuous fibers, maintaining continuity, andpreserving orientations of fibers have proven to be major obstacles forconventional additive manufacturing techniques.

Described herein are composite feedstock strips for additivemanufacturing and methods of forming such strips. These compositefeedstock strips include continuous unidirectional fibers. Morespecifically, the fibers extend parallel to each other and to theprincipal axes of the strip. These feedstock strips may be produced fromhigh grade composite plies and films without introducing voids or othertypes of defects.

A composite feedstock strip is formed by laminating a layup of one ormore fiber-containing plies and one or more of resin plies. The positionof these plies in the layup is used to control distribution of thefibers and other materials within the resulting strip. Furthermore, theorientation of all fiber-containing plies in the layup is such that allfibers in this layup are unidirectional. After lamination, the laminatedsheet is slit into multiple composite feedstock strips. The slitting isperformed along the direction parallel to the fibers in these strips. Assuch, the continuity of the fibers is preserved. The proposed methods offorming composite feedstock strips are low cost, applicable to a widerange of resin materials thermoplastic materials) and fiber materials,and can be easily tuned to produce different amounts and/or distributionof fibers within the feedstock strips. The feedstock can be used forfused deposition modeling (FDM) additive manufacturing technologies toproduced composite parts. Composite feedstock strips include continuousunidirectional fibers and may be also referred to as reinforcedfeedstock strips or, more specifically, continuous fiber reinforcedfeedstock strips or rods.

Any planar plies may be used to form a layup, including but not limitedto specialty plies, such aerospace grade fiber-containing plies, and thelike. Furthermore, different layup arrangements may be used to achievedifferent distribution of fibers and other materials within resultingfeedstock strips thereby opening doors for new and unique configurationsof composite feedstock strips. :Furthermore, this wide range of materialoptions and arrangement options allow economical processing with minimalfiber disruption or buckling as well as continuous equipment runtime.Various continuous processing techniques, such as roll-to-rollprocessing, may be used for individual operations or a combination ofmultiple operations, such as a combination of forming a layup andlaminating the layup as further described below.

A layup may be formed from continuous rolls of plies. One of these rollsmay include a fiber-containing ply. The fibers in this ply may becontinuous and extend in the direction of roll windings. In someembodiments, multiple rolls of the same or different fiber-containingplies may be used to form the same layup. Other plies may be resinplies, which may be free from fibers. A method may be a continuousprocess in which rolls containing one or more fiber-containing plies andone or more resin containing plies unwind, and the plies arecontinuously fed into processing equipment (e.g., a laminator) forconsolidating all plies of the layup into a laminated sheet. In someembodiments, a slitter may also be a part of this continuous process.The slitter cuts the laminated sheet into individual composite feedstockstrips, which could be formed into rolls for compact storage andshipping. This continuous process may also include a liquefier, whichchanges the cross-sectional profile of the composite feedstock strips.For example, the strips may have the square profile after slitting andthen the circular profile after passing through the liquefier. Finally,additive manufacturing may also be a part of the continuous process.

In some embodiments, composite feedstock strips are coated. Addition ofthe coating after the composite feedstock strips are slits may be usedto change their cross-sectional profile, to add material on the outersurface that is suitable for additive manufacturing or particularapplication, and/or to use composite feedstock strips that have higherconcentrations of continuous unidirectional fibers (and have a higheroverall fiber concentration even accounting for the coating layer, whichmay be free from fibers). For example, changing the cross-sectionalprofile by redistributing some material on the outer surface may berequire a substantial amount of a fiber free material on the surface toavoid fiber disturbance. Some limitations may be imposed on thecomposition of these fiber free materials and/or processing conditionsused during redistribution. On the other hand, coating of the slitstrips with a material provides new material options, such as materialshaving fillers, to form uniform coating layers, and other features. Insome embodiments, redistribution may be combined with coating.

During the additive manufacturing, the composite feedstock strips areused to form composite parts, usually parts with complex geometricalshapes. This continuous processing is generally faster and morecontrolled (e.g., better fiber orientation control) than conventionaldiscrete processing, especially when some operations are performed byhand. One having ordinary skills in the art would understand that notall processing operations described above need to be performed. Forexample, composite feedstock strips may be used without changing theircross-sectional profiles. In some embodiments, the strips may be laiddown and consolidated into a part using thermoplastic compositeplacement technique. Furthermore, additive manufacturing may be a partof a different process altogether. Finally, grouping of these processingoperations may differ and may not necessarily be a part of one largegroup. For example, layup formation and lamination may be a part of onegroup. A roll of the laminated sheet may be formed after completing alloperations in this group. This roll may be then slit into compositefeedstock strips during a slitting operation belonging to another group.Yet another processing group may include cross-sectional profilechanging operations.

Overall, provided composite feedstock strips have low cost and highquality and may be formed from a wide range of composite materials, in awide range of configurations, as well as a wide range of cross-sectionalsizes and profiles. These feedstock strips can be produced in largevolumes to supply the needs of a continuous fiber reinforced additivemanufacturing market. Comparable feedstock made directly usingthermoplastic composite pultrusion processes have not been able toefficiently produce small diameter rod material particularly in thehigher performance thermoplastic materials suitable for high endapplications.

Examples of Composite Feedstock Strips and Forming Thereof

FIG. 1A is a process flowchart corresponding to method 100 of formingcomposite feedstock strips for additive manufacturing, in accordancewith some embodiments. Method 100 may commence with forming a layupduring operation 102 followed by laminating the layup during operation110. A laminated sheet is formed during operation 110 and later slitinto composite feedstock strips during operation 120. In someembodiments, the cross-sectional shape of the composite feedstock stripsis changed during optional operation 130. This operation 130 may involveheating the composite feedstock strips during optional operation 132and/or redistributing material during optional operation 134.Redistributing the material may be performed without impacting relativeorientations of fibers in the composite feedstock strips. In someembodiments, method 100 may involve performing additive manufacturingduring optional operation 140. The composite feedstock strips may beconsumed during this operation to form a composite part. Each of theseoperations will now be described in more detail with reference tovarious figures illustrating components at various stages of method 100,equipment used to perform the described operations, and test samples.

Referring to operation 102, which involves forming a layup, the layupformed during this operation may include one or more fiber containingplies and one or more resin plies. As further described below, the oneor more resin plies may not include fibers. Even if fibers are includedin the one or more of resin plies, these fibers are different from theone or more fiber containing plies, which include continuousunidirectional fibers.

Referring to FIG. 2A illustrating one example of layup 200, thisparticular layup includes four fiber containing plies 204 a-204 d andsix resin plies 202 a-202 f. The number, thickness, and arrangement offiber containing plies 204 and resin plies 202 may be used to control,at least in part, the cross-sectional distribution of materials withinlaminated sheet 210 shown in FIG. 3A (formed by laminating layup 200).This material may be maintained, at least to some extent, in compositefeedstock strips 220, which are formed by slitting laminated sheet 210as further described below. Furthermore, the number and the thickness ofplies 202 and 204 may be used to control thickness 210 a of laminatedsheet 210, which in turn controls the cross-sectional dimension ofcomposite feedstock strips 220.

Resin plies 202 used to form layup 200 may be free from fibers. Allcontinuous unidirectional fibers may be provided in fiber containingplies 204. In some embodiments, resin plies 202 may include other typesof fillers, such as particles and/or short multidirectional fibers.Referring to FIG. 2A, in some embodiments, at least one outer ply 208 aof layup 200 is resin ply 202. More specifically, both outer plies 208 aand 208 b may be resin plies 202. All other plies of layup 200,including fiber containing plies 204 and, in some embodiments, otherresin plies 202, are disposed between outer plies 208 a and 208 b. Insome embodiments, multiple outer plies on each side of layup 200 areresin plies 202. The example presented in FIG. 2A illustrates two resinplies 202 a and 202 b on one side of layup 200 and two resin plies 202 eand 202 f on the other side of layup 200. This type of arrangement maybe used to ensure that sufficiently thick surface portions of layup 200and then of laminated sheet 210 and eventually of composite feedstockstrips 200 are free from fibers to allow changing cross-sectional shapesof composite feedstock strips 200.

In some embodiments, resin plies 202 comprise one or more materialsselected from the group consisting of polyethersulfone (PES),polyphenylenesulfide (PPS), polyetheretherketone (PEEK),polyetherketoneketone (PEKK), polyetherimide (PEI), and thermoplasticpolyimide (TPI). More specifically, one or more resin plies 202 comprisepolyethersulfone (PES). All resin plies 202 forming the same layup 202may have the same composition. Alternatively, different resin plies 202forming the same lay up may have different compositions.

In some embodiments, fiber containing plies 204 comprise one or morematerials selected from the group consisting of polyethersulfone (PES),polyphenylenesulfide (PPS), polyetheretherketone (PEEK),polyetherketoneketone (PEKK), polyetherimide (PEI), and thermoplasticpolyimide (TPI). These materials may be referred to matrix resins andshould be distinguished from the resin of resin plies 202. Morespecifically, fiber containing plies 204 may comprisepolyetherketoneketone (PEKK).

The resin used in fiber containing plies 204 may be the same ordifferent than the resin used in resin plies 202. For example, resinplies 202 may comprise polyethersulfone (PES), while fiber containingplies 204 may comprise polyetherketoneketone (PEKK). In someembodiments, resin plies 202 may include polyetherketoneketone (PEKK),while fiber containing plies 204 may comprise polyphenylenesulfide(PPS). In some embodiments, resin plies 202 may includepolyetherketoneketone (PEKK), while fiber containing plies 204 maycomprise polyetherketoneketone (PEKK).

One or more resins used in fiber containing plies 204 and in resin plies202 may be thermoplastic resins. In sonic embodiments, one or moreresins used in fiber containing plies 204 and in resin plies 202 mayinclude a thermoset resin. The thermoset resin, if used, may be combinedwith one or more thermoplastic resins (e.g., used as a filler).Furthermore, when the thermoset resin is used, fiber containing plies204 and/or resin plies 202 containing this resin may be heated, forexample, above the glass transition temperature of that thermoset resin.

The thickness of each resin ply 202 may be between about 0.001 inchesand 0.020 inches or, more specifically, between 0.002 inches and 0.010inches. The thickness of each fiber containing ply 204 may be betweenabout 0.003 inches and 0.015 inches or, more specifically, between 0.005inches and 0.010 inches.

Referring to FIG. 2B, continuous fibers 206 of fiber containing plies204 may be any suitable fibrous components, such as glass (S-type orE-type), quartz, aramid, carbon fibers, carbon nanotubes, orcombinations thereof. Substantially all (e.g., more than 90%) fibers 206within each fiber containing ply 204 are continuous and oriented in aunidirectional arrangement as, for example, shown in FIG. 2Billustrating fibers 206 extending along the Y axis. The unidirectionalarrangement may be also referred to as 0/0 arrangement. Specifically,all fibers 206 in all fiber containing plies 204 forming layup 200 areparallel to each other. One having ordinary skills in the art wouldunderstand that the term parallel allows for some tolerance such as lessthan about ±5° or even less than about ±2°.

Other types of fiber orientations (not unidirectional) may interferewith subsequent slitting of laminated sheet 210 but may nonetheless beapplicable for forming composite feedstock strips 220 in accordance withthe methods described herein. One of ordinary skill in the art wouldrecognize that the type, cross-sectional dimensional, amount of fibers206 within fiber containing plies 204, as well as the type of the matrixresin utilized in fiber containing plies 204 and the resin used in resinplies 202 may vary, based on numerous factors, including cost and theultimate desired physical and mechanical properties of compositefeedstock strips 220.

In some embodiments, all fiber containing plies 204 forming layup may beinitially provided in rolls, e.g., prepreg tapes. Fibers 206 in thesefiber containing plies 204 may extend along the winding direction ofthese rolls. When multiple fiber containing plies 204 are used all pliesare precisely oriented with respect to each other in layup 200 to ensurethat all fibers 206 in layup 200 are parallel to each other(unidirectional).

In some embodiments, forming layup 200 is performed in a roll-to-rollprocess. Referring to FIG. 7A, fiber containing plies 204 a and 204 band resin plies 202 a and 202 b may be unrolled from respective rolls702 and form layup 200 upon entering preheating zone 704. Thesecontinuous sheet forming processes, as with roll-to-roll handling, canbe performed in a double belt press, roll pultrusion machine, orcontinuously compressed in molding machines.

Returning to FIG. 1A, after completing operation 102, method 100continues with laminating layup 200 during operation 110. During thisoperation, laminated sheet 210 is formed. Specifically, the material ofall resin sheets 202 and all fiber containing sheets 204 may beconsolidated during this operation. At the same time, the unidirectionalorientation of fibers 206 may be preserved. Fibers 206 may move closerto each other during this operation or otherwise change theirorientation within the cross-section. For example, when multiple fibercontaining plies 204 are used to form layup, fibers 206 in one of thesefiber containing plies 204 may move closed to fibers in another one ofthese fiber containing plies 204. In some embodiments, the orientationof fibers 206 provided in each of fiber containing plies 204 may remainsubstantially the same. For example, fiber containing plies 204 may bepreviously consolidated. Alternatively, one layup 200 is formed,relative orientation of fibers 206 may remain the same during laminationoperation 110.

Continuing with FIG. 1A and operation 110, this laminating operation 110may involve heating and compressing layup 200. In some embodiments,operation 110 may be performed in a continuous manner (e.g., in aroll-to-roll manner) using, for example, apparatus 700 shown in FIG. 7A.Specifically, apparatus 700 may include preheating zone 704 forpre-heating layup to a lamination temperature. One haying ordinaryskills in the art would understand that the lamination temperature maydepend on the resins used in fiber containing plies 204 and resin plies202, thickness of these plies, and other process parameters. In general,the lower temperature limit should be sufficient to ensure meltconsolidation of different plies forming layup 200 and to a certainextent flow of materials (other than fibers 206) forming layup. On theother hand, the upper temperature limit may need to be controlled tomaintain orientation of fibers 206 during consolidation of the pliesinto laminate sheet 210 and to prevent thermal degradation.

During operation 110, heated layup 200 may be fed from preheating zone704 into lamination zone 710, which may be also referred to as aconsolidation zone. In lamination zone, layup 200 is consolidated toform single integrated laminate sheet 210. As layup 200 moves forwardthrough lamination zone 710, it may be continuously heated at leastthrough initial part of consolidation zone 710.

One example of laminated sheet 210 is shown in FIG. 3A. Thickness 210 a(shown as T_(total) in FIG. 3A) of laminated sheet 210 extends betweentwo surfaces 213 and 215 of that sheet. In some embodiments, thethickness of laminated sheet 210 (T_(total)) is less than about 0.060inches or, more specifically, less than about 0.050 inches or even lessthan about 0.040 inches. It should be noted that the thickness oflaminated sheet 210 (T_(total)) determines the cross-section ofcomposite feedstock strips 220 as further described below with referenceto FIG. 4B. Also shown in FIG. 3A is center plane 217 of laminated sheet210 positioned at equal distances between two surfaces 213 and 215 ofthat sheet.

In some embodiments, the volumetric fraction of fibers 206 withinlaminated sheet 210 varies throughout the thickness of laminated sheet210. For purposes of this document, a volumetric fraction is defined asa ratio of the volume of one component (e.g., fibers 206) to the overallvolume of the structure containing this component. When the volumetricfraction is discussed with a reference to the cross-section of astructure, the volumetric fraction may be presented as a ratio ofcross-sectional areas (i.e., a ratio of the cross-sectional area of thecomponent in question to the overall cross-sectional of the entirestructure). The variability of the volumetric fraction of fibers 206within laminated sheet 210 may be attributed to the arrangement of oneor more fiber containing plies 204 and one or more of resin plies 202 inlayup 210 as well as the composition of each ply.

The example of laminated sheet 210 presented in FIG. 3A includes twosurface portions 212 and 216. Specifically, surface portion 212 formsfirst surface 213 of laminated sheet 210, while surface portion 216forms second surface 215. Both portions 212 and 216 may be substantiallyfree from fibers 206. Center portion 214 may include all fibers 206 oflaminated sheet 210. For clarity, center portion 214 is disposed betweentwo surface portions 212 and 216. This is an example of concentratingfibers 206 around center plane 217 of laminated sheet 210. This type ofdistribution may be achieved by forming surface portions 212 and 216from resin plies 202 only. In this example, resin plies 202 are freefrom fibers. At the same time, center portion 214 may be formed fromvarious one or more fiber containing plies 204. In some embodiments, oneor more resin plies 202 may be also used to form center portion.Referring to the example of layup 200 shown in FIG. 2A, first surfaceportion 212 may be formed from resin plies 202 a and 202 b, while secondsurface portion 216 may be formed from resin plies 202 e and 202 f.Center portion 214 may be formed from fiber containing plies 204 a-204 das well as resin plies 202 c and 202 d. This arrangement and number offiber containing plies 204 and resin plies 202 is selected to achieve adesired thickness of center portion 214 as well as distribution offibers within center portion 214 and laminated sheet 210 overall.

Referring to FIG. 3A, a ratio of thickness 212 a of surface portion 212,which may be free from fibers 206, to total thickness 210 a of laminatedsheet 210 (T_(portion)/T_(total)) may be between about 5% and 45% or,more specifically, between about between about 10% and 30%. Thisfiber-free portion 212 allows changing the cross-sectional profile ofcomposite feedstock strips 220 without disturbing fibers 206 as furtherdescribed below with reference to FIGS. 5B-5C.

FIG. 8A illustrates a cross-sectional image of a test laminate sheet(having ply arrangements similar to the example shown in FIG. 2A anddescribed above). This cross-sectional image of FIG. 8A illustrates anactual distribution of fibers throughout the cross-section of the testsample. Specifically, this test laminate sheet was prepared using thefollowing layup: two PES plies/carbon-PEKK ply/PES ply/two carbon-PEKKplies/PES ply/carbon-PEKK ply/two PES plies. The carbon-PEKK pliesincluded above 60% of carbon fibers and each had a thickness of about0.0054 inches. The PES plies were each 0.005 inches in thickness. Thehorizontal white patches are cross-sections of fibers 206 provided by inthe carbon-PEKK plies. There are four distinct groupings of these whitehorizontal patches, each grouping corresponding to a separatecarbon-PEKK ply. The patches are clearly positioned away from surfaces213 and 215 of this test laminate sheet corresponding to the modelpresented in FIG. 3A and described above.

Referring to FIGS. 3A-B and FIG. 8A, the volumetric fraction of fibers206 within laminated sheet 210 is greater at center plane 217 oflaminated sheet 210 than at one or both surfaces 213 and 215.Specifically, FIG. 3B illustrates one example of volumetric fractionprofile 219 based on the location along the thickness direction (the Zaxis). This figure identifies surface portions 212 and 216, being freefrom fibers, and center portion 214, containing all fibers 206. Sincesurface portions 212 and 216 are free from fibers, the volumetricfraction in these portions is at a zero level. In some embodiments,volumetric fraction profile 219 is symmetric with respect to centerplane 217 of laminated sheet 210 as, for example, shown in FIG. 3B. Thissymmetric profile may be achieved by a symmetric position of plies inlayup 200, such as in the example illustrated in FIG. 2A.

In some embodiments, the average of volumetric fraction of fibers 206within entire laminated sheet 210 is between about 1% and 60% on averageor, more specifically, between about 10% and 50% or even between about20% and 40%. This characteristic may be also referred to as a totalfiber loading. However, unlike most of conventional composite materials,laminated sheet 210 has uneven distribution of fibers 206.

Returning to FIG. 1A, method 100 may continue with slitting laminatedsheet 210 into composite feedstock strips 220 during operation 120.During this operation laminated sheet 210 is turned into compositefeedstock strips 220. Slitting may be performed using one of waterjetslitting, rotational cutting, pressure wheel slitting, or roll slitting.Furthermore, slitting the laminated sheet 210 into the compositefeedstock strips 220 is performed in a roll-to-roll process.

Referring to FIGS. 3D and 4A, slitting may be performed along direction230 parallel to all fibers 206 with laminated sheet 210. As such,slitting does not disturb the continuity of fibers 206, i.e., does notcut fibers. Cross-sectional profile 221 of each of composite feedstockstrips 220 formed during operation 120 may be a square as, for example,shown in FIGS. 4A-4C. More generally, cross-sectional profile 221 may bea rectangle. This type of profile 221 is a result of slitting in thedirection substantially perpendicular to surfaces 213 and 215 oflaminated sheet 210. This square or rectangular profile may be laterconverted into a round profile, e.g., a circular profile or an ovalprofile.

Referring to FIGS. 4B and 4C, all fibers 206 in each composite feedstockstrip 220 are parallel to primary axis 223 of that strip. For purposesof this document, primary axis 223 is defined as an axis extending alongthe longest dimension of composite feedstock strip 220, such as itslength 220 c as shown in FIG. 4C. As described above, all fibers 206 areparallel in laminated sheet 210 prior to its slitting. Furthermore,slitting is performed along the direction parallel to the fibers. As aresult, all fibers 206 remain parallel (as defined above) to each otherin composite feedstock strip 220 and extend parallel to primary axis 223of the strip. This continuous and unidirectional orientation of fibersresults in superior tensile strength and other mechanicalcharacteristics of composite feedstock strip 220. For example, tensilestrength of over 90 ksi have been measured in some representative teststrips.

At the same time, composite feedstock strip 220 may be bendable indirections perpendicular to its primary axis 223. This bendingcapability is provided by the unidirectional orientation of fibers 206and, in some embodiments, by uneven distribution of fibers 206 withincomposite feedstock strip 220. Specifically, FIGS. 6A and 6B illustratea cross-section of strip bent 90°. Second surface portion 226experiences a sharper bend radius (R₁) than the bend radius (R₂) ofcenter portion 224. At the same time, surface portions 226 may be freefrom fibers 206 and, as a result, may be more malleable than centerportion 204. The fiber distribution allows composite feedstock strip 220to be coiled in small diameter packages for storage, shipment, andsubsequent use in additive manufacturing.

In some embodiments, fibers 206 may have an average length of at least100 feet or even at least about 1000 feet in composite feedstock strips220. This reflects the continuity aspect of fibers in compositefeedstock strips 220. At the same time, the principal cross-sectionaldimension 220 d of composite feedstock strips 220 after reforming, asfor example shown in FIG. 5C, may be less than about 0.060 inches or,more specifically, less than about 0.050 inches or even less than about0.040 inches. This small cross-sectional dimension 220 d may be neededfor additive manufacturing. If the cross-sectional profile of compositefeedstock strip 220 is a circle, then its principal cross-sectionaldimension 220 d is the diameter of the circle as shown in FIG. 5C.However, if the cross-sectional profile of composite feedstock strip 220is a square, then its principal cross-sectional dimensions are width 220b and thickness 220 a, which are the same as shown in FIG. 4B.

Returning to FIG. 1A, method 100 may involve changing thecross-sectional profile of each composite feedstock strip duringoptional operation 130. For example, the cross-sectional profile of eachcomposite feedstock strip may be changed from being a square (afterslitting of the laminated sheet) to a circle or a hexagon. Currently,the nozzles used for additive manufacturing have round profiles to helpwith directional changes while applying materials. As such, having roundor similar (close to being round) cross-sectional profile of compositefeedstock strips 220 may help during additive manufacturing. However,additive manufacturing techniques may be developed to use otherfeedstock strips with other types of cross-sectional profiles. Thecurrent methods allow forming many different types of profiles withoutdisturbing orientation of continuous fibers or their continuity.

Operation 130 may involve heating (block 132 in FIG. 1A) compositefeedstock strip 220 and redistributing (block 134 in FIG. 1A) material229 away from corners 228 of cross-sectional profile 221 a as shown inFIGS. 5B and 5C. Specifically, FIG. 5B illustrates composite feedstockstrip 220 disposed within form 510. The cross-sectional profile of form510 may be round. However, the diameter of form 510 at this location(identified as A-A and referring to FIG. 5A) is greater than thediameter of final cross-sectional profile 221 b identified with a dashline in FIG. 5B. At this location, corners 228 of composite feedstockstrip 220 contact form 510. It should be noted that corners 228 extendoutside of the boundary of final cross-sectional profile 221 b and anysuch outside material will be brought within the boundaries (e.g., byfilling voids) during operation 130. In some embodiments, material 229redistributed away from the corners 228 is free from fibers 206.

This redistribution of the outside material during operation 130 may beperformed without substantial impact on the material that is within theboundary of final cross-sectional profile 221 b. Specifically, theposition of continuous fibers 206 within composite feedstock strip 220is retained during operation 130 as, for example, illustrated with FIGS.5B and 5C showing static cross-sectional profile of center portion 224of composite feedstock strip 220. This may be achieved by selectiveheating of corners 228 of composite feedstock strip 220 attributable tothe direct contact of corners 228 with heated form 510. The remainingportion of composite feedstock strip 220 may be heated less than cornersdue to the heat insulating nature of the materials forming compositefeedstock strip 220 and speed with which operation 130 is performed.Specifically, during operation 130, center portion 224 comprisingcontinuous fibers 206 may have a lower temperature than material 229being redistributed away from corners 228. As noted above, material 229may be free from continuous fibers 206.

Changing cross-sectional profile operation 130 may be performed usingliquefier 500, one example of which is shown in FIG. 5A. Liquefier 500may include form 510 with a tapered cross-sectional profile and heater512 for heating form 510. Because continuous fibers 206 of compositefeedstock strip 220 are unidirectional and continuous, compositefeedstock strip 220 can be easily fed through liquefier 500 withoutclogging it (which is a common problem with chopped continuous fibersand particles used as structural supports in composites). Furthermore,composite feedstock strip 220 may substantially retain its tensilestrength while being fed through liquefier 500 again due to thecontinuous nature of continuous fibers 206, which simplifies handlingcomposite feedstock strip 220 during operation 130.

In some embodiments, operation 130 is not performed. Composite feedstockstrips 220 having a rectangular or a square profile may be used forsubsequent processing. Method 100 may also involve performing 140additive manufacturing using composite feedstock strips 220.

FIG. 1B is a process flowchart corresponding to another example ofmethod 100. This example involves a coating operation performed on slitstrips and, as such, composite feedstock strips formed using this methodmay be referred to as coated composite feedstock strips 520. Variousexamples of coated composite feedstock strips 520 are shown in FIGS.5G-5K and further described below with references to these figures andthe coating operation. Unless specifically noted, a composite feedstockstrip identified with numeral 220 may be referred to as an uncoatedcomposite feedstock strip to distinguish it from coated compositefeedstock strip 520. Uncoated composite feedstock strips 220 areconverted into coated composite feedstock strips 520 during the coatingoperation.

Returning to FIG. 1B, many operations of method 100 of forming coatedcomposite feedstock strips 520 may be similar to correspondingoperations of method 100 of forming uncoated feedstock strips 220, whichis described above with reference FIGS. 1A. Various references to theflowchart in FIG. 1A and the corresponding description above are madewhen appropriate.

The main difference between the flowcharts in FIGS. 1A and 1B is coatingoperation 136 of the later flowchart. Specifically, coating operation136 may be performed after forming uncoated feedstock strips 220. By wayof reminder, uncoated feedstock strips 220 may be formed by slitting alaminated sheet during operation 120.

In some embodiments, coating operation 136 eliminates the need to changethe cross-sectional shape of the composite feedstock strip duringoperation 130 by redistributing at least some material on the surface ofthe strip. In these embodiments, operation 130 is not performed.Alternatively, when operation 130 is performed, coating operation 136may be performed before or after operation 130. In other words,cross-sectional shape changing operation 130 may be performed either onuncoated composite feedstock strips 220 (followed by the coating) or oncoated composite feedstock strips 520.

Another difference between flowcharts presented in FIGS. 1A and 1B or,more specifically, a difference between materials used in these twoexamples may be fiber distribution in uncoated composite feedstockstrips 220. This fiber distribution corresponds to the one in alaminated sheet used to form composite feedstock strips 220. In theexample of method 100 presented in FIG. 1B, the coating operation may beused for cross-sectional shape changing instead of rearranging portionsof composite feedstock strips 220. Specifically, composite feedstockstrips 220 may remain undisturbed while changing the cross-sectionalshape, e.g., by adding the coating material onto the outer surface ofcomposite feedstock strips 220. As such, the entire cross-section ofcomposite feedstock strips 220 may include continuous fibers. Nofiber-free portions are needed in composite feedstock strips 220 in thisparticular example since composite feedstock strips 220 remainundisturbed. The continuous fibers may be positioned near all surfacesof uncoated composite feedstock strips 220. The laminated sheet used toform uncoated feedstock strips 220 may be formed from fiber containingplies only and may not include external resin sheet.

Referring to FIG. 1B, method 100 may comprise forming laminated sheet210 during optional operation 104. Operation 104 may involve forminglayup 200 during operation 102 followed by laminating layup 200 duringoperation 110. Some examples of operations 102 and 110 are describedabove with reference to FIG. 1A. Examples of layup 200 and laminatedsheet 210 are illustrated in FIGS. 2A-2D, 3A and 3D. Layup 200 mayinclude one or more fiber containing plies 204 and, in some embodiments,resin plies 202. Resin plies 202 may be used as outer plies 208 a and208 b as, for example, shown in FIG. 2A).

In some embodiments and unlike the example of method 100 described abovewith reference to FIG. 1A, layup 200 formed in accordance with method100 of FIG. 1B may be formed from only fiber containing plies 204. FIG.2C illustrates one examples of such layup 200 including fiber containingplies 204 a-204 d. It should be noted that both outer plies 208 a-208 bin this example are also fiber containing plies. In this example, thecross-sectional change of composite feedstock strips 202 may be achievedby coating these composite feedstock strips 202 rather thanredistributing the material.

Alternatively, layup 200 may include one or more resin plies in additionto one or more fiber containing plies. However, outer plies 208 a-208 bmay be fiber containing plies. FIG. 2D illustrates an example of suchlayup where resin plies 202 a and 202 b are positioned inside the stackwhile fiber containing ply 204 a serves as one outer ply 208 a and fibercontaining ply 204 d serves as another outer ply 208 b.

As stated above, the coating operation may be used for cross-sectionalshape changing. At least no material redistribution may be performed onuncoated composite feedstock strip 220 and no fiber-free material isneeded on its surfaces. As such, outer plies 208 a-208 b of layup 200may contain continuous fibers.

In these embodiments, the volumetric fraction of the continuous fiberswithin layup 200 and later in laminated sheet 210 may be constantthroughout the thickness as, for example, shown in FIG. 3C.Specifically, FIG. 3C illustrates fiber volumetric fraction profile 219throughout the thickness of laminated sheet 210 (or uncoated compositefeedstock 200). However, as the coating layer is formed, thiscross-sectional distribution changes as continuous fibers are generallynot provided in this coating layer.

When one or more resin plies are used to form layup 200, these plies maybe free from continuous fibers and, in some embodiments, free from otherfillers. Because some plies have continuous fibers while other plies donot, the volumetric fraction of the continuous fibers within layup 200and later with laminated sheet 210) varies throughout. One such exampleis described above with reference to FIG. 3B where the volumetricfraction of the continuous fibers may be greater at center portion 214than at both surface portions 212 and 216. Comparing volumetric fractionprofiles 219 in FIGS. 3B and 3C, one having ordinary skill in the artwould understand that a higher loading of continuous fibers may bepossible when the number of resin plies is reduced or even completelyeliminated from layup 200.

Returning to FIG. 1B, method 100 of forming coated composite feedstockstrips 520 may comprise slitting laminated sheet 210 into compositefeedstock strips during operation 120. It should be noted that thecomposite feedstock strips formed during operation 120 are stilluncoated. Various examples of slitting operation 120 are described abovewith reference to FIG. 1A and are also shown in FIGS. 3D and 4A.

Laminated sheet 210 and, as a result, slit feedstock strips 220 maycomprise resin 207 and continuous fibers 206 extending parallel to eachother along primary axis 223 of strip 220 (i.e., in the Y direction) asschematically shown FIGS. 4B and 4C. The slitting may be performed alongthe direction parallel to all continuous fibers 206 thereby preservingcontinuity of fibers 206.

In some embodiments, the concentration of continuous fibers 206throughout the cross section of uncoated composite feedstock strips 220is at least about 30% by volume or even at least about 40%, at leastabout 50%, or even at least about 60%. Such a high concentration offibers 206 can provide excellent mechanical properties, such as atensile strength in the direction of fibers 206. This concentration maybe achieved by eliminating portions free from continuous fibers 206,such as surface portions 222 and 226 shown in FIG. 4B. As describedabove, fiber-free surface portions 222 and 226 are needed, when acoating is not used, to redistribute material in these portions withoutdisturbing the fibers. Since fibers 206 are not added or removed duringslitting operation 120, the fiber concentration of laminated sheet 210and uncoated composite feedstock strips 220 may be the same. Thisconcentration may be controlled during fabrication of laminated sheet210 or, more particularly, while forming layup 200 during operation 102.

In some embodiments, the cross-sectional profile of uncoated compositefeedstock strips 220 is selected from the group consisting of arectangle, a square, a circle, and a trapezoid. Some of these examplesare shown in FIGS. 5D-5F. The circular cross-sectional profile may beformed from an initially rectangular or square profile by redistributingmaterial before coating as further described below with reference toFIGS. 5L and 5M. The trapezoidal cross-sectional profile may be formed,for example, when a wedged shape slitting tool pushes some of thematerial in the direction of the slit. It should be noted that forming acircular profile from starting with the trapezoidal cross-section byredistributing material on the top and bottom surfaces may bechallenging. Forming a coating layer may help to overcome thesechallenges.

Returning to FIG. 1B, method 100 may proceed with coating outer surface225 of composite feedstock strips 220 during operation 136. The coatingoperation may involve a cross-head extrusion coating technique, powdercoating, and solution-based coating technique. FIG. 7B illustrates anexample of apparatus 720 for coating composite feedstock strips 220 andforming coated composite feedstock strips 520.

During operation 136, coating layer 522 is formed on outer surface 225as schematically shown in FIGS. 5D and 5G. Coating layer 522 may includea resin. This resin may be the same as or different from the resin ofuncoated composite feedstock strips 220, which may be a part of pliesforming layup 200. For clarity, the resin of uncoated compositefeedstock strips 220 may be referred to as first resin 207, while theresin of coating layer 522 may be referred to as second resin 523 (referto FIG. 5G, for example). In some embodiments, one of first resin 207and second resin 522 comprises one or more materials selected from thegroup consisting of polyethersulfone (PES), polyphenyl sulfide (PPS),polyetheretherketone (PEEK), polyetherketoneketone (PEKK),polyetherimide (PEI), and thermoplastic polyimide (TPI). For example,first resin 207 and second resin 523 may be both polyetherketoneketone(PEKK).

In some embodiments, the material used for coating layer 522 comprises afiller in addition to second resin 523. The filler may be selected fromthe group consisting of fibers, particles, and flakes. For example, thefiller may comprise discontinuous fibers, which are different from thecontinuous fibers of the sheet and later of the composite feedstockstrips at least based on their aspect ratio. The filler may be selectedfrom the group consisting of a heat sensitive additive, a mineralreinforcement, a thermal stabilizer, an ultraviolet (UV) stabilizer, alubricant, a flame retardant, a conductive additive, a pigment, andvarious combinations thereof. In one example, the filler is a heatsensitive additive. In the same of another example, the filler is amineral reinforcement. In the same of another example, the filler is athermal stabilizer. In the same of another example, the filler is anultraviolet (UV) stabilizer. In the same of another example, the tilleris a lubricant. In the same of another example, the filler is a flameretardant. In the same of another example, the filler is a conductiveadditive. In the same of another example, the filler is a pigment.

In some embodiments, the thickness of coating layer 522 is uniform. Thistype of coating layer may be also referred to as a conformal coating.For example, the thickness variation may be less than 20% or even lessthan 10%. In these embodiments, the cross section of coated compositefeedstock strip 520 may represent a scaled up variation of the crosssection of composite feedstock strips 220 prior to its coating as, forexample, schematically shown in FIGS. 5H and 5K.

In some embodiments, the cross section of composite feedstock strips 220or, more specifically, the cross-sectional profile of compositefeedstock strips 220 remains the same during coating operation 136. Thisshape retention is schematically shown in FIGS. 5D and 5G. In theseembodiments, the coating process does not disturb composite feedstockstrips 220.

Despite the cross-sectional profile of composite feedstock strips 220remaining the same during coating operation 136, the cross-sectionalprofile of coated composite feedstock strips 520 may be different thanthat of uncoated composite feedstock strips 220. For example, uncoatedcomposite feedstock strip 220 may have a rectangle, square, or trapezoidprofile as described above. Coated composite feedstock strip 520 formedfrom this uncoated composite feedstock strip 220 may have a circularprofile or an oval profile as, for example, schematically shown in FIGS.5D and 5G.

Various examples of coated composite feedstock strip 520 are shown inFIGS. 5G-5K. In some embodiments, the cross-sectional profile of coatedcomposite feedstock strips 520 may be the same as of uncoated compositefeedstock strips 220, see, e.g., FIG. 5M or 5K. One having ordinaryskill in the art would understand that the principal dimension of thecross-sectional profile will still increase in these examples. Ingeneral, the cross-sectional profile of coated composite feedstock strip520 is selected from the group consisting of an oval, a circle, arectangle, a square, and a rounded corner rectangle, and a roundedcorner square.

In some embodiments, prior to coating operation 136, method 100 mayinvolve changing the cross-sectional profile of uncoated compositefeedstock strip 220 during operation 130. This example is schematicallyshown in FIGS. 5L-5N. Specifically, FIG. 5L illustrates uncoatedcomposite feedstock strip 220 having a square cross-sectional shape.This shape may be a result of slitting operation 120, for example.During operation 130 this square cross-sectional shape is changed byredistributing material of uncoated composite feedstock strip 220. Thisoperation is described in more detail with reference to FIG. 1A. FIG. 5Millustrates still uncoated composite feedstock strip 220 having acircular cross-sectional shape after this operation. Subsequently, thiscircular composite feedstock strip 220 may be coated thereby formingcoated composite feedstock strip 520 as, schematically, shown in FIG.5N.

In some embodiments, method 100 may involve changing the cross-sectionalprofile of coated composite feedstock strip 520. In other words, shapechanging operation 130 is performed after coating operation 136. Thisexample is schematically shown in FIGS. 5O-5Q. Specifically, FIG. 5Oillustrates uncoated composite feedstock strip 220 having a squarecross-sectional shape. This square composite feedstock strip 220 may becoated thereby forming coated composite feedstock strip 520 as,schematically, shown in FIG. 5P. During operation 130 this squarecross-sectional shape is changed by redistributing material of coatinglayer 522 of coated composite feedstock strip 220. Without beingrestricted to any particular theory, it is believed that the approachshown in FIGS. 5O-5Q may be less impactful to continuous unidirectionalfibers 206 than the approach shown in FIGS. 5L-5N.

Characteristics of various embodiments of coated composite feedstockstrips for use in additive manufacturing were explored using anillustrative analysis. In this analysis, a coating of neat resin (freeof fibers) or a coating of resin containing 30 wt % discontinuous fibersis applied to a square laminate core made up of fiber-containing plies.This core has a constant volumetric fraction of continuous fibers ofabout 60% throughout its thickness. FIG. 8B is a plot of coating volumefraction for a circular coating as a function of the laminate thicknessand the thickness of the coating. For a square laminate core, thelaminate width is equal to the laminate thickness, and the laminatethickness increases discretely based on the number of fiber-containingplies used in the layup. The laminate thicknesses described in FIG. 8Bcorrespond to 4-9 fiber-containing plies. The relative size of datapoints corresponds to the final coated rod diameter, with solid linesconnecting embodiments with the same diameter equal to that given to theleft of the series. The coating thickness is given by the minimumcoating thickness measured radially from the corner of the laminate coreas shown in the inset figure. Dotted lines connect embodiments with thesame minimum coating thickness equal to that listed to the right of theseries. To narrow the range of possibilities to those of technicalsignificance for practical manufacture and use, the followingconstraints were placed on these embodiments and those to follow withother coating cross sectional shapes: (1) the laminate thickness isgreater than 0.030 in. for practical slitting operations; (2) theminimum coating thickness at any location in the cross section is 0.004in. or larger to allow for manufacturing tolerance in the coatingoperation; (3) the largest dimension in the cross section should be lessthan 0.070 in. to allow the feedstock to enter a liquifier with an inletdiameter of 0.070 in.; and (4) to ensure that no continuous fiberinterference occurs with the wall of the liquefier, a minimum of 0.001in. of coating material is maintained between the continuousfiber-containing laminate core and the wall of the liquefier after anyshape conversion. With these constraints, the solution space forsatisfactory embodiments is denoted by the triangular region highlightedin FIG. 8B. Relatively large volume fractions of coating are required,from 51-65%. This translates to 20-28% fiber content by volume in thefinal coated rod for a neat resin coating and 36-40% for a coating with30 wt % discontinuous fibers.

The fiber content in the final coated feedstock is plotted in FIG. 8C asa function of the laminate thickness, final coated feedstockcross-sectional shape, and coating material. The solution spaceshighlighted satisfy the four constraints listed previously for squarelaminate cores with coatings of the following shapes: circular, square,and rounded corner square. The solution spaces corresponding to coatingswith neat resin and with 30 wt % discontinuous fibers are given by theregions with solid borders and those with dotted borders, respectively.Circular coatings do not require shape conversion in the liquefierduring additive manufacturing, whereas square and rounded squarecoatings require shape conversion to circular in the liquefier or priorto the liquefier. The cross-sectional area of the exit of the liquefieris equal to the cross-sectional area of the incoming coated feedstock toensure steady and stable flow through the liquefier is maintained.

The solution spaces for circular coated feedstocks in FIG. 8C correspondto the triangular region highlighted in FIG. 8B. The maximum laminatecore thickness is limited to ensure the final coated feedstock can befed reliably into the 0.070 in. liquefier entrance, and the fibercontent achievable is limited by the amount of coating needed to ensurethe minimum coating thickness is at least 0.004 in. for practicality ofthe coating operation.

The volume fraction of coating could conceivably be decreased andtherefore the overall fiber content increased by coating with a squarecoating and allowing the shape to change in the liquefier as depicted inFIG. 5O-5Q. The square coating solution space is limited to smalllaminate thicknesses to allow the feedstock to enter the 0.070 in.liquefier entrance. The range of fiber content for the coated feedstockresults from varying the coating thickness, as measured from the side ofthe laminate core, with higher fiber content resulting from thinnercoating thicknesses. A minimum of 0.005 in. coating thickness isrequired to ensure that the overall coating volume fraction is largeenough that at least 0.001 in. coating material is maintained betweenthe corners of the laminate core and the wall of the liquefier after theshape conversion to circle. This coating thickness yields the maximumfiber content of 34% for a neat resin coating and 44% for a 30 wt %discontinuous fiber-filled coating.

The solution space may be expanded to larger laminate thicknesses whilestill maximizing the overall fiber content by using a rounded cornersquare coating, as that depicted in FIG. 5I. Larger laminate corethicknesses are able to be used because the largest dimension in thecross section, the diagonal distance from opposite corners, is reduced.Cases where the largest dimension is 0.070 in. are also accepted becauseless contact area with the liquefier walls at the entrance allows formore reliable feed compared to circular coatings which have full contactwith the walls. For a given laminate thickness, the fiber content in thecoated feedstock may be increased by using thinner coatings, as measuredfrom the side of the laminate, with corners rounded such that thediagonal distance is 0.070 in. The maximum achievable fiber content isalso plotted for each laminate thickness where the coating thickness atthe rounded corner, measured radially from the corner of the laminate,is held at the minimum acceptable value of 0.004 in. for practicalcoating operation. With these maximum rounded corners, the coatingthickness measured from the side of the laminate should be 0.005 in.,0.006 in., and 0.007 in. for laminate thicknesses of 0.033 in., 0.039in., and 0.044 in., respectively, yielding fiber contents of 34-35% byvolume using a neat resin coating and 44% by volume using a coating with30 wt % discontinuous fibers.

Examples of Aircraft and Methods of Fabricating and Operation Aircraft

The illustrated embodiments provide a novel fabrication method offorming composite feedstock strips with continuous unidirectionalorientations of continuous fibers and tailored distribution of thesecontinuous fibers throughout the cross-section of the strips.Furthermore, these methods provide for different cross-sectionalprofiles and/or dimensions of the strips. Continuous processing used inthese methods not only increases processing throughput but also provideshigh level of control of various characteristics of the compositefeedstock strips. The embodiments find applicable uses in a wide varietyof potential applications, including for example, in the aerospaceindustry. The disclosed method is ideally suited for additivemanufacturing of parts having complex geometries, such as brackets, clipsupports, link levers, or more generally any irregular crosssections-structures, which are currently formed from metal (e.g., lugs,end fittings). The parts should be generally distinguished from partshaving simple (e.g., linear) geometries such as beams (such asnon-varying cross sections). The disclosed method is also suited forone-of-a-kind, customized, or very limited part runs with non-varyingcross section, which could be fabricated using additive manufacturing.

Examples of the present disclosure may be described in the context ofaircraft manufacturing and service method 1100 as shown in FIG. 9 andaircraft 1102 as shown in FIG. 10. During pre-production, method 1100may include specification and design (block 1104) of aircraft 1102 andmaterial procurement (block 1106). During production, component andsubassembly manufacturing (block 1108) and system integration (block1110) of aircraft 1102 may take place. Composite feedstock strips may beformed and used in additive manufacturing during one of these steps,e.g., specification and design (block 1104) of aircraft 1102, materialprocurement (block 1106), component and subassembly manufacturing (block1108), and system integration (block 1110) of aircraft 1102. Thereafter,aircraft 1102 may go through certification and delivery (block 1112) tobe placed in service (block 1114). While in service, aircraft 1102 maybe scheduled for routine maintenance and service (block 1116). Routinemaintenance and service may include modification, reconfiguration,refurbishment, etc. of one or more systems of aircraft 1102.

Each of the processes of method 1100 may be performed or carried out bya system integrator, a third party, and/or an operator (e.g., acustomer). For the purposes of this description, a system integrator mayinclude, without limitation, any number of aircraft manufacturers andmajor-system subcontractors; a third party may include, withoutlimitation, any number of vendors, subcontractors, and suppliers; and anoperator may be an airline, leasing company, military entity, serviceorganization, and so on.

As shown in FIG. 10, aircraft 1102 produced by method 1100 may includeairframe 1118 with a plurality of high-level systems 1120 and interior1122. Examples of high-level systems 1120 include one or more ofpropulsion system 1124, electrical system 1126, hydraulic system 1128,and environmental system 1130. Any number of other systems may beincluded. Although an aerospace example is shown, the principlesdisclosed herein may be applied to other industries, such as theautomotive industry. Accordingly, in addition to aircraft 1102, theprinciples disclosed herein may apply to other vehicles, e.g., landvehicles, marine vehicles, space vehicles, etc.

Apparatus(es) and method(s) shown or described herein may be employedduring any one or more of the stages of method 1100. For example,components or subassemblies corresponding to component and subassemblymanufacturing (block 1108) may be fabricated or manufactured in a mannersimilar to components or subassemblies produced while aircraft 1102 isin service (block 1114). Also, one or more examples of theapparatus(es), method(s), or combination thereof may be utilized duringproduction stages (block 1108 and block 1110), for example, bysubstantially expediting assembly of or reducing the cost of aircraft1102. Similarly, one or more examples of the apparatus or methodrealizations, or a combination thereof, may be utilized, for example andwithout limitation, while aircraft 1102 is in service (block 1114)and/or during maintenance and service (block 1116).

Conclusion

Different examples of the apparatus(es) and method(s) disclosed hereininclude a variety of components, features, and functionalities. Itshould be understood that the various examples of the apparatus(es) andmethod(s) disclosed herein may include any of the components, features,and functionalities of any of the other examples of the apparatus(es)and method(s) disclosed herein in any combination, and all of suchpossibilities are intended to be within the spirit and scope of thepresent disclosure.

Many modifications of examples set forth herein will come to mind to oneskilled in the art to which the present disclosure pertains having thebenefit of the teachings presented in the foregoing descriptions and theassociated drawings.

Therefore, it is to be understood that the present disclosure is not tobe limited to the specific examples illustrated and that modificationsand other examples are intended to be included within the scope of theappended claims. Moreover, although the foregoing description and theassociated drawings describe examples of the present disclosure in thecontext of certain illustrative combinations of elements and/orfunctions, it should be appreciated that different combinations ofelements and/or functions may be provided by alternative implementationswithout departing from the scope of the appended claims. Accordingly,parenthetical reference numerals in the appended claims are presentedfor illustrative purposes only and are not intended to limit the scopeof the claimed subject matter to the specific examples provided in thepresent disclosure.

1. A method of forming coated composite feedstock strips for additivemanufacturing, the method comprising: slitting a sheet into compositefeedstock strips, the sheet comprising a first resin and fibersextending parallel to each other within the sheet; slitting beingperformed along a direction parallel to all of the fibers within thesheet, and coating an outer surface of the composite feedstock stripswith a material comprising a second resin thereby forming the coatedcomposite feedstock strips comprising a coating layer disposed over thecomposite feedstock strips.
 2. The method of claim 1, wherein the fibersextending parallel to each other within the sheet are continuous fibers.3. The method of claim 1, wherein a distribution of the fibersthroughout a cross section of the composite feedstock strips is uniform.4. The method of claim 1, wherein a concentration of the fibersthroughout a cross section of the composite feedstock strips is at leastabout 40% by volume.
 5. The method of claim 1, wherein a cross sectionof the composite feedstock strips remains same while coating the outersurface of the composite feedstock strips with the material.
 6. Themethod of claim 1, wherein a thickness of the coating layer on the outersurface of the composite feedstock strips is uniform.
 7. The method ofclaim 1, wherein the material used for coating layer further comprises afiller selected from the group consisting of fibers, particles, andflakes.
 8. The method of claim 7, wherein the filler comprisesdiscontinuous fibers.
 9. The method of claim 7, wherein the filler isselected from the group consisting of a heat sensitive additive, amineral reinforcement, a thermal stabilizer, an ultraviolet (UV)stabilizer, a lubricant, a flame retardant, a conductive additive, and apigment.
 10. The method of claim 1, wherein coating is performed usingone of a cross-head extrusion coating technique, powder coating, or asolution-based coating technique.
 11. The method of claim 1, wherein across-sectional profile of the composite feedstock strips is selectedfrom the group consisting of a rectangle, a square, and a trapezoid, andwherein a cross-sectional profile of the coated composite feedstockstrips is selected from the group consisting of an oval, a circle, arectangle, a square, and a rounded corner rectangle, and a roundedcorner square.
 12. The method of claim 1, wherein one of the first resinand the second resin comprises one or more materials selected from thegroup consisting of polyethersulfone (PES), polyphenylenesulfide (PPS),polyetheretherketone (PEEK), polyetherketoneketone (PEKK),polyetherimide (PEI), and thermoplastic polyimide (TPI).
 13. The methodof claim 1, wherein the first resin and the second resin are same. 14.The method of claim 13, wherein the first resin and the second resin areboth polyetherketoneketone (PEKK).
 15. The method of claim 1, furthercomprising, prior to slitting the sheet, forming a layup comprisingfiber containing plies and laminating the layup thereby forming thesheet.
 16. The method of claim 15, wherein all sheets of the layup arethe fiber containing plies.
 17. The method of claim 1, furthercomprising, prior to slitting the sheet, forming a layup comprising oneor more fiber containing plies and one or more of resin plies andlaminating the layup thereby forming the sheet.
 18. The method of claim17, wherein a volumetric fraction of the fibers within the laminatedsheet varies throughout a thickness of the laminated sheet.
 19. Themethod of claim 18, wherein the volumetric fraction of the fibers withinthe laminated sheet is greater at a center of the laminated sheet alongthe thickness of the laminated sheet than at one of surfaces of thelaminated sheet. 20-21. (canceled)
 22. A coated composite feedstockstrip, the coated composite feedstock strip comprising: a compositefeedstock strip comprising a first resin and fibers extending parallelto each other within the sheet; and a coating layer comprising a secondresin and disposed on an outer surface of the composite feedstock stripand forming a shell around composite feedstock strip, the coating layercomprising a second resin. 23-33. (canceled)